DMSO as recommended by CLSI.

Try chloroform to dissolve your oils..worked for me to test antifungal activity

Chloroform is very toxic for microorganisms (at least fungi). Hexane is less toxic and a good solvent for some oils (for example, fatty acids). But DMSO is the best.

beside DMSO I think that ethyl acetate or ethyl lactate could work as well...

what about Tween 80

what about Tween 80 in culture media and ethanol mixed with EOs in the same time...

My background being in chemistry and pharmacology, I am curious about a few things related to this discussion and would welcome being enlightened.

a) For oils to have practical application with regards to their antimicrobial activity, the target microorganisms would need to be lipophilic, would they not? If so, then:

b) Would I be right in assuming that lipophilic bacteria not being pathogenic, the main application would be in relation to fungi?

c) If the microorganisms (bacteria or fungi) are lipophilic, would the culture medium not have to also be lipophilic (or at least partially so, e.g. emulsions)? If so, oils being liquid already, why would there be a need for a vehicle/solvent? What would happen if the oil was blended with the culture medium or simply applied to the medium in the same way that aqueous solutions are to agar plates? My thinking being that anything that interferes with solubility characteristics of a compound may also affect its bioactivity, itself a further complication.

I would welcome having my misconceptions pointed out.

There are several cellular targets, among them are lipophilic targets as the plasma membrane and the outer membrane of Gram-negative bacteria, and others are hydrophilic as the bacterial cell wall, plasma proteins, DNA ...

For testing the antimicrobial activity we have two commonly used techniques: diffusion discs on solid media and dilution methods (in liquid media). The problem arises in the second technique where we have several choices non-standardized. I inform you that the solvents used for testing the activity of the essential oils are not toxics at concentrations well defined by researchers.

Has anyone tried homogenizing the EOs directly into BHI without solvents?  

DMSO (can be used as a dispersing solvent) is recommended for testing the antimicrobial activity of essential oils. You can further download the pdfs of the files attached herewith to obtain certain interesting informations.